Methods of cleaning papermaking fabrics are known. Traditional methods of cleaning papermaking fabrics include low and high pressure showering of wet end fabrics (i.e., fabrics used in forming and press sections of the papermaking process) and dry end fabrics (i.e., dryer fabrics). Wet end fabric cleaning may include, for example, two in-place showers used for each wet end fabric, namely, (1) a chemical application shower (hereinafter, “the wet application shower”), which typically operates at less than 100 psi pressure and is used for chemical application for cleaning the fabrics, both during machine operation (hereinafter “on the fly cleanings”) and during nonoperational intervals (hereinafter “down batch cleanings”), and (2) a high pressure continuous cleaning oscillating shower bar, which typically runs from 150-550 psi (water only) (hereinafter, “the wet oscillating shower”). Water temperatures for the application shower and the oscillating shower are typically in the range of about 100-150° F.
Cleaning chemicals are typically introduced through either (i) the application shower, (ii) the oscillating shower, or (iii) both (i) and (ii). Chemicals are typically applied at either a highly caustic level (i.e., a pH of 9-13) or a mid-level acidic level (i.e., a pH of 3-5), each of which is labeled as hazardous. U.S. Pat. No. 7,597,782 assigned to DuBois Chemicals, Inc. (Sharonville, Ohio, USA) (hereinafter, “DuBois”) describes using alternating acidic and caustic chemical batches delivered through an oscillating shower bar using an application pressure of from about 150-250 psi. Dubois sells a metering and delivery system called the DUPULSE™ system for this type of cleaning application.
For press fabrics, on the fly batch washes are generally done multiple times daily. Either (i) sheet handling problems or (ii) high Uhle box vacuum readings on the paper machines or (iii) other operational issues such as web breaks or poor moisture profiles force the operation to periodically stop the paper machine and perform a thorough down batch cleaning to alleviate the problem, again using either caustic or acidic cleaners. In addition, if the machine's operation is interrupted for other reasons, the personnel normally use that opportunity to perform a down batch cleaning of the fabrics.
Typically, press fabric life averages about 4 to 8 weeks using this type of cleaning system. At the end of the fabric life, the fabrics are removed from the machine and replaced with new fabrics.
In dryer fabric cleaning operations on many modern or updated paper machines, the early (i.e., first, second, and possibly more) dryer groups of the dryer section may be equipped with a single dryer fabric configured to serpentine around the top and bottom drying cylinders, commonly known as unirun(s) or unorun(s) fabrics. Later dryer groups, or those not equipped with a unirun fabric, are typically and more conventionally configured whereby the top and bottom drying cylinders are equipped with separate top and bottom fabrics. Because the early dryer groups are operated at a higher temperature (i.e., about 200° F.; significantly higher than the preceding press section), the paper web is subjected to a thermal shock that causes contaminants contained within the paper web to precipitate out and onto the dryer fabric(s); causing the contaminants to adhere to the surface of the dryer fabric. For this reason, these early dryer groups pose significant cleaning challenges to the papermaker. Further, given that the operation in this section is specifically aimed at removing moisture dryer sections, by necessity avoid adding water to this stage of the operation.
There are three common types of showers in the unirun section:
(1) A low pressure chemical application shower (hereinafter, “the dryer application shower”) that typically operates at less than 100 p.s.i. and is used when the paper machine is not in operation (i.e., down batch cleaning), and traditionally uses a caustic based chemical (i.e., usually one per unirun fabric).
(2) An oscillating, high pressure (500+ p.s.i.) shower (hereinafter, “the dryer oscillating shower”) used when the paper machine is not in operation (i.e., down batch cleaning) and without the addition of cleaning chemicals. When present, the oscillating shower is often used for aiding in rinsing the fabric and lifting the contaminant during a down batch chemical cleaning (i.e., usually one per unirun fabric, but not always present).
(3) A very high pressure (i.e., 2500+ adjustable psi) continuous traversing needle shower (hereinafter, “the dryer needle shower”) equipped with some type of evacuation mechanism (e.g., often vacuum assisted) for removing the water and contaminants from the fabric. This shower is less frequently present given that it is a fairly high cost capital item. When present, they are most frequently found in the early dryer groups. Traditionally, this type of shower has never been used to deliver chemical cleaners to the fabric. Various manufacturers make these types of showers.
On the fly cleanings are typically not done in the unirun section. Down batch cleaning cycles vary greatly depending on contaminant loading, but can be necessary as frequently as once per week. Web breaks, sheet handling issues, poor moisture profile, embossing (marking) of the paper web (lower quality), and high steam/energy requirements are some of the reasons for stopping the machine to clean the dryer fabric(s). Using the chemical application shower, caustic based washing chemicals are typically used (i.e., a pH 9-13). Typically, extra rinsing time is required because the caustic, if left on the fabric, causes degradation of the fabric yarns and seam, and can drastically decrease the life of the fabric. Because of the hazardous nature of the cleaners, chemical vendors are often on site to handle the cleanings.
There exists a need in the art for less hazardous and more efficient methods of cleaning papermaking fabrics.